Description
The message "Option is in use as a DHCP setting" may be seen when attempting to set as DHCP option 138.

Solution
A workaround is to use the following CLI commands to enter DHCP option 138.
For example, to configure option 138 with value 192.168.1.1, the command line would be:
config system dhcp server
edit <dhcpservername>
set option1 138 C0A80101
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Technical Tip: Using DHCP server 'options' on a FortiGate